The hunt for planets like our own has come up with a striking discovery: There's a planet about the same size as Earth in the nearby Alpha Centauri system, and it's the closest planet found outside our solar system.
"Close," of course, is a relative term. No one's getting there anytime soon: The newly found planet, which orbits a star called Alpha Centauri B, is about 4 light-years, or 23.5 trillion miles, away.
Based on its mass, the planet is a rocky world and not gaseous, said Xavier Dumusque of the Geneva Observatory in Switzerland. He and his colleagues published the findings in the journal Nature.
Scientists used an instrument called the High Accuracy Radial velocity Planet Searcher. HARPS, located at the European Southern Observatory's La Silla Observatory in Chile, searches for planets outside our solar system using a radial velocity method.
Here's how that works: When a planet orbits a star, the star wobbles back and forth slightly because of the planet's gravitational force. Scientists are looking at how fast the star is moving toward or away from Earth. They measure this through the wavelength of the light from the star, which gets shifted if there's a planet present (this is the Doppler effect).
The newfound planet is unlikely to harbor life, or at least life as we know it, Dumusque said.
It's located extremely close to its parent star -- Earth has a 365-day orbit around our sun, and this other planet orbits its star in only three days. Temperatures on the surface could be in the area of 1300 degrees Fahrenheit, scientists estimate. Rather than solid, the surface is likely to be lava.
But there is hope for life in that neighborhood: Small-mass planets like the one orbiting Alpha Centauri B are usually not alone with their sun, Dumusque said. Often there are other planets in the system, farther away from the parent star.
The next step would be to continue monitoring the shifts in light from the star, looking for other planets. Time is of the essence, however: As Alpha Centauri B and another star, Alpha Centauri A, move closer to each other, finding any planets in the area will become more difficult. Proxima Centauri, the closest star to Earth apart from the sun, may be related to this binary system.
When a planet similar to the Earth is detected, the next step is to start characterizing its atmosphere, looking for elements such as carbon and oxygen that are familiar to life on our own planet.
Finding those will be difficult with present technology, however, as HARPS has limitations. A high-tech tool is slated to come online in 2016 -- it's called ESPRESSO (Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations).
About 800 planets have been confirmed to exist outside our solar system, in addition to nearly 2,000 planet candidates found with the Kepler mission.
Before you get too excited about the planet orbiting Alpha Centauri B, keep in mind that it has not yet been confirmed by a second group of astronomers. But Dumusque says there's only a 1 in 10,000 chance that this is not a planet.
Sara Seager, professor of planetary science at Massachusetts Institute of Technology, was ecstatic about the news.
In order to study planets outside our solar system in detail, she said, these exoplanets need to be around the nearest stars. And Alpha Centauri has captured the human imagination for decades, she points out, even appearing in the film "Avatar." Its proximity is a motivator for sending probes, and even astronauts, to a different star system.
"I think the reality of a planet around the nearest star and the promise of more is a game changer," she said in an e-mail.
If other planets orbiting Alpha Centauri B are the same mass and size as the new planet, they will be hard to detect, she said. It would take eight more years of this same kind of data, using similar technology, to find a planet of similar mass in a "habitable zone" -- in other words, at a distance from the star that would permit life.
"Hopefully Nature has provided a planet that is more easily detected," she wrote.